Method for aromatic hydrocarbon recovery



Sept. 9, 1969 35 GRAFF ETAL 3,466,346

METHOD FOR AROMATIC HYDROCARBON RECOVERY Filed May 29, 1967 Q 1 1 E GW m fivusuxt 2352 IN VE/VTORS Richard R. OeGraff BY: Marl/n W. Parga A T TORNEYS United States Patent US. Cl. 260-674 5 Claims ABSTRACT OF THE DISCLOSURE Method for extracting aromatic hydrocarbons from a suitable feedstock such as catalytic reformate using a solvent such as sulfolane. The extracted aromatics are recovered using a combination of extractive distillation followed by fractionation.

Background of the invention This invention relates to the recovery of aromatic hydrocarbons via the solvent extraction of aromatic components from a suitable feedstock. It particularly relates to the recovery of aromatic hydrocarbons from the extract phase from such a solvent extraction operation. It specifically relates to the distillation of a solvent phase, such as sulfolane having aromatic hydrocarbons dissolved therein, in order to recover the aromatic hydrocarbons such as benzene, toluene, and xylene therefrom.

It is known in the art that a conventional process for the recovery of high purity aromatic hydrocarbons from various feedstocks, including catalytic reformate, is liquidliquid extraction utilizing a solvent such as diethylene glycol or sulfolane, each of which has high selectivity for the aromatic hydrocarbon components contained in the feedstock. Typically, in the practice of such prior art process, a hydrocarbon feed mixture is contacted in an ex raction zone with an aqueous solvent composition which selectively dissolves the aromatic component from the hydrocarbons feedstock, thereby forming a raffinate phase comprising one or more non-aroma ic hydrocarbons and an extract phase comprising solvent having aromatic components dissolved therein. The extract phase is then separately distilled yielding an overhead distillate containing only a portion of the extracted aromatic component, a sidecut fraction comprising aromatic hydrocarbons, and a bottoms fraction comprising lean solvent suitable for reuse in the extraction zone. Frequently, to prevent losses of the solvent, the raffinate phase is washed with water in a washing zone in order to remove solvent from the raffinate phase.

Also, not infrequently, the extract phase is subjected to extractive distillation in order to remove a contaminating quantity of non-aromatic hydrocarbons from the extract phase. This extractive distillation operation is normally performed in order to make possible the recovery of nitration grade aromatic hydrocarbons such as benzene and toluene. Therefore, a typical prior art process for the recovery of aromatic hydrocarbons encompasses a solvent extraction step, an extractive distillation step, and a final distillation step for the recovery of the aromatic hydrocarbons from the solvent phase.

In the extractive distillation operation it is common practice to add to the extract phase considerable quantities of additional solvent so that the relative volatilities between a non-aromatic hydrocarbon component and an aromatic hydrocarbon component are substantially increased in order to effectuate an almost complete separation between the two via distillation. This, of course, requires a distillation column of some complexity utiliz- Patented Sept. 9, 1969 ing large quantities of utilities, such as steam for heat input, in order to properly perform the distillation. However, the extractive distillation column is severely limited in the amount of heat input which is possible because care must be taken to minimize the quantity of aromatics in the overhead fraction which would represent a loss in yield of desirable aromatic hydrocarbons by virtue of adding inefliciencies to the extraction operation to which the overhead stream is normally returned. Accordingly, the extractive distillation operation achieves a balance between the desire to remove non-aromatic hydrocarbons from the aromatic hydrocarbons and the desire to maximize the recovery of the aromatic hydrocarbons.

In similar fashion, the operation of the aromatic recovery column is one of achieving proper thermal balance. It would be desirable to have the feed to the aromatic recovery column as high as possible in temperature so that a minimum amount of reboiler heat may be added to the column. It is also desirable to control the temperature of the feed since relatively small changes in temperature have a large effect on the heat balance of the column.

Summary of the invention It is therefore an object of this invention to provide a method for the recovery of aromatic hydrocarbons from the extract phase of a solvent extraction operation in a more facile and economical manner.

It is another object of this invention to provide a method for fractionating a sulfolane solvent having dissolved therein aromatic and non-aromatic hydrocarbons in a facile and economical manner.

Thus, according to one embodiment of this invention there is provided a method for recovering aromatic hydrocarbons from the extract phase from a solvent extraction zone which comprises the steps of: (a) introducing an extract phase containing solvent having aromatic hydrocarbons dissolved therein, said extract also contaminated with non-aromatic hydrocarbons, into a first distillation zone in the presence of a hereinafter specified added solvent stream; (b) withdrawing from said first zone an upper stream comprising said contaminants, an intermediate vapor sidecut fraction comprising aromatic hydrocarbons, and a lower stream comprising solvent and aromatic hydrocarbons; (c) introducing said intermediate stream and said lower stream directly into a second distillation zone maintained under distillation condition; (d) withdrawing from said second zone a distillate fraction comprising aromatic hydrocarbons, an intermediate sidecut fraction comprising solvent contaminated with aromatic hydrocarbons, and a bottoms fraction comprising solvent suitable for reuse in said extraction zone; said, (e) returning at least a portion of said sidecut fraction of step (d) to step (a) as said added solvent.

It is noted from the hereinabove brief description of the present invention, relative to the prior art, that significant economies of operation are achieved by the expedient of withdrawing from both the extractive distillation column and the aromatic recovery distillation column a sidecut fraction which is utilized in a novel manner. In the extractive distillation column the sidecut fraction is a vapor stream which is passed as a vapor directly into the aromatic recovery column. In the aromatic recovery column, the sidecut fraction is a lean solvent stream containing small amounts of aromatic hydrocarbons which are then returned to the extractive distillation column as the added solvent phase. It was discovered that by operating in this manner, the capital expense and operating expense, particularly for the aromatic recovery column, were significantly decreased and that the small amount of aromatic hydrocarbons in the solvent being returned to the extractive distillation operation did not in any way decrease the ultimate yield of aromatic hydrocarbons, nor did it influence in any way the efliciency of the removal of non-aromatic hydrocarbons in the extractive distillation operation.

The hydrocarbon feed mixture which may be separated by the improved method of the present invention comprises many different aromatic-nonaromatic mixtures. Typically, feedstocks applicable to the solvent extraction step include hydrocarbon distillate fractions (usually boiling within or near the gasoline boiling range) of natural gasoline or straight-run petroleum distillates and especially comprises reformed naphthas which are rich in aromatic compounds and which are particularly valuable as a source of mononuclear aromatic hydrocarbons, such as benzene, toluene, and xylene. Thus, the desired aromatic hydrocarbons which are ultimately recovered in the practice of this invention may comprise benzene, toluene, benzene and toluene, toluene and xylene; and benzene, toluene, and xylene. In each case, however, it is understood that the feedstock contains non-aromatic hydrocarbons as well as aromatic hydrocarbons; that is, the aromatic extract from the solvent extraction step comprises solvent having aromatic hydrocarbons dissolved therein, but this extract is also contaminated with a small amount of nonaromatic hydrocarbons. The process of the present invention is specifically directed to the separation of hydrocarbon feed mixtures comprising benzene and toluene and to feed mixtures comprising toluene and xylene as the aromatic hydrocarbon. Typically, the feedstocks of the present invention, as charged to the aromatic extraction step, will contain from about to about 60% by weight aromatic hydrocarbons; although, aromatic concentrations as high as 95% by weight may be used in some cases. The quantity of added solvent is directly proportional to the aromatic content of the feed.

Solvent compositions which may be utilized in the practice of the present invention are those selected from the classes which have high selectivity for aromatic hydrocarbons. These aromatic selective solvents generally contain one or more organic compounds containing in their molecule at least one polar group such as hydroxyl, amino, cyano, carboxyl, or nitro radical. In order to be effective, the organic compounds of the solvent composition having the polar radical must have a boiling point substantially greater than the boiling point of water which preferably is included in the solvent composition for enhancing its selectivity, and in general, must also have a boiling point substantially greater than the end boiling point of the aromatic component to be extracted from the hydrocarbon feed mixture.

Organic compounds suitable for use as part of the solvent composition preferably are selected from the group of those organic containing compounds which include the aliphatic and cyclic alcohols, cyclic monomeric sulfones, the glycols and glycol ethers, as well as the glycol esters and glycol ether esters. The monoand poly-alkylene glycols in which the alkylene group contains from 2 to 3 carbon atoms, such as ethylene glycol, diethylene glycol, triethylene glycol, and tetraethylene glycol, propylene glycol, dipropylene glycol, and tripropylene glycol, as well as the methyl, ethyl, propyl, and butyl ethers of the glycol hydroxyl groups and the acetic acid esters thereof, constitute a satisfactory class of organic solvents useful in admixture with water as the solvent composition for use in the present invention. An illustrative glycol comprises triethylene glycol.

Additionally, excellent results may be obtained utilizing the cyclic monomeric sulfone, such as tetrahydrotriophene-l,l-dioxide. Still further, an organic compound particularly useful in the practice of this invention is a sulfolane which may be made by condensing a conjugated diolefin with sulfur dioxide and then subjecting the resulting product to hydrogenation, alkylation, hydration and/ or other substitution or addition reactions. Typically, or-- ganic compounds belonging to the sulfolane class are 2- sulfolene, Z-methylsulfolane, 2,4dimethylsulfolane, 2,4- dimethyl-4-sulfolane, methyl-3-sulfonyl ether, ethyl-3-sulfonyl sulfide, and others.

The apparatus embodied in the practice of the present invention may be any conventional or convenient type known to those skilled in the art. Also, the o erating con ditions suitable for the practice of this invention are conventional and well known to those skilled in the art, with exception of the precise temperatures and pressures for operating the extractive distillation column and the aromatic recovery column according to the teachings of this invention. In any event, from the teachings presented and from a general knowledge of the art those skilled in the art will be able to choose the proper operating conditions to. achieve the benefits ascribed to the practice of the present invention. The amount of solvent composition utilized should be at least sufficient to dissolve the constituent to be extracted. It may be desirable to use a considerable excess over the theoretical amount of solvent composition necessary, especially when maximum purity and maximum recovery of the aromatic hydrocarbons are desired. Usually, in the extraction step the solvent composition to feed ratios will range from about 1:1 to about 20:1 by volume, preferably, from about 5:1 to about 15:1 by volume. A summary of the conditions necessary for the practice of the sulfolane type of operation may be found in Petroleum Refiner, volume 38, No. 9, September 1959, pages 185- 192, the entire disclosure of which is incorporated herein by reference.

The solvent extraction step, as previously mentioned, is well known and may utilize apparatus of any type suitable for effecting counter-current contact between two liquid phases, at least partially, but not wholly miscible with each other and wherein the relatively more dense solvent may be brought into intimate contact with the relatively less dense hydrocarbon phase. Thus, the extraction zone which produces the solvent extract which is used as feedstock to the practice of the present invention may comprise a packed column or may contain a series of horizontal plates through which the liquid solvent flows in dispersed form and in counter-current flow relationship to the ascending hydrocarbon stream.

The invention may be more fully understood with reference to the appended drawing.

Description of the drawing The appended drawing is a schematic representation of apparatus for practicing one embodiment of the invention.

Referring now to the drawing, an extract phase from a conventional solvent extraction zone is passed into the system via line 10. The extract phase has aromatic hydrocarbons dissolved therein and due to inherent inefiiciencies of any commercial solvent extraction step this extract phase is also contaminated with a small amount of non-aromatic hydrocarbons. The extract feed in line 10 is mixed with an added solvent stream from line 11 the source of which is more fully described hereinafter. The combined extract feed stream plus added solvent is passed via line 12 into extractive distillation column 13 which is maintained under distillation conditions.

Operating conditions in distillation column 13 are conventional in that sutficient heat must be added to the column in order for separation to take place between the non-aromatic hydrocarbons and the aromatic hydrocarbons dissolved in the solvent. Typical operating conditions for distillaiton column 13 for use with, say, sulfolane solvent include a pressure from mm. Hg to 15 p.s.i.g., an overhead temperature from F. to 330 F., and a bottoms temperature from F. to 355 F., typically, about 350 F.

Operating under these conditions a non-aromatic hydrocarbon upper stream is withdrawn from column 13 via line 14. A lower stream comprising solvent and aromatic hydrocarbons is withdrawn, preferably, from the bottom of column 13 via line 16 and passed directly into aromatic recovery distillation column 17. A critical feature of this invention is the withdrawal of a vapor sidecut fraction from distillation column 13 via line 15. The locus for withdrawal is selected so that the material in line 15 contains a concentration of aromatic hydrocarbons and is of sufiicient purity from an aromatics standpoint so that no contaminating non-aromatic hydrocarbons will appear in the distillate fraction from distillation column 17. The vapor sidecut fraction from distillation column 13 is now passed via line 15 into distillation column 17. As a practical mode of operation, the material injected into column 17 in liquid phase is introduced at a locus, for example, of two (2) fractionating trays higher in the column than the locus for the introduction of the vapor phase stream in line 15. In some cases, however, it may be preferable to introduce this fraction at a locus lower than the vapor stream locus or to admix the streams and feed them together into column 17 Distillation column 17 is operated in conventional manner for the separation of aromatic hydrocarbons from the selective solvent. Depending upon the volatility characteristics of the solvent, the operating conditions are chosen so that extremely high purity aromatic hydrocarbons may be withdrawn from distillation column 17 via line 18. Lean solvent suitable for reuse in extraction process is withdrawn from distillation column 17 via line 19. However, in the practice of this invention, a sidecut fraction comprising relatively lean solvent containing perhaps a minor amount of aromatic hydrocarbons is also withdrawn from distillation column 17 via line 11 and returned to admixture with the incoming extract feed stream in line as hereinabove described. By operating in this manner the stripping load in the bottom of distillation column 17 is considerably lessened in that the amount of material in line 11 does not have to pass down the column and be returned to the extraction Zone with the lean solvent in line 19. a

Typical operating conditions for distillation column 17 when utilizing sulfolane as the solvent and desiring to recover benzene and toluene as the aromatic hydrocarbons include an overhead temperature from 120 F. to 306 F., a bottoms temperature from 300 F., to 350 F., and a column pressure from 90 mm. Hg to 760 mm. Hg. The sidecut fraction in line 11 is withdrawn generally at a temperature from 300 F. to 350 F. Those skilled in the art from a general knowledge and from the teachings presented herein will know how to choose the proper operating conditions to effectuate recovery of high purlty aromatic hydrocarbons from the solvent.

One of the advantageous features discovered in the practice of the embodiments of this invention is the achievement of a considerably elevated temperature 1n the bottom of distillation column 13. By utilizing the vapor sidecut withdrawal features from this extractive distillation column, a considerable increase in the temperature of the bottoms may be achieved without increasing the loss of aromatic hydrocarbons in the non-aromatic hydrocarbon fraction removed from the column via line 14. By operating at a considerably increased and essentially constant temperature at the bottom of the extractive distillation column considerably less heat is required in final aromatic recovery distillation column 17. This eliminates the necessity to oversize the recovery column reboiler to compensate for variations in feedstock composition. Minimizing the recovery column reboiler duty also facilitates the use of a desirable stab-in type unit.

Preferred embodiment From the teachings presented hereinabove, the preferred embodiment of this invention includes the method previously mentioned wherein the solvent comprises sulfolane.

Another preferred embodiment includes the broad method previously mentioned wherein the lower stream from the first distillation zone is introduced into the second distillation zone at a locus below the locus for introducing said vapor sidecut stream.

A still further preferred embodiment of this invention provides an improvement in a process for the recovery of aromatic hdyrocarbons dissolved in sulfolane solvent wherein non-aromatic hydrocarbons are contaminating the extract phase from the solvent extraction zone, said contaminants being removed by extractive distillation of the extract phase, which comprises withdrawing from the extractive distillation column a vapor sidecut stream com- .prising a concentrate of aromatic hydrocarbons and liquid bottoms stream comprising solvent; passing both withdrawn streams into an aromatic recovery distillation column maintained under distillation conditions; withdrawing from the aromatic recovery column a distillate fraction comprising aromatic hydrocarbons, a bottoms fraction comprising lean solvent, and a sidecut fraction comprising solvent containing aromatic hydrocarbons; and, returning said sidecut solvent fraction to the extractive distillation column.

What is claimed is:

1. Method for recovering hydrocarbons from the extract phase from a solvent extraction zone which comprises the steps of:

(a) introducing an extract phase containing solvent having aromatic hydrocarbons dissolved therein, said extract also contaminated with non-aromatic hydrocarbons, into a first distillation zone in the presence of a hereinafter specified added solvent stream;

(b) withdrawing from said first zone an upper stream comprising said contaminants, an intermediate vapor sidecut stream comprising aromatic hydrocarbons, and a lower stream comprising solvent and aromatic hydrocarbons;

(c) introducing said intermediate stream and said lower stream directly into a second distillation zone maintained under distillation conditions;

((1) withdrawing from said second zone a distillate fraction comprising aromatic hydrocarbons, an intermediate sidecut fraction comprising solvent contaminated with aromatic hydrocarbons, and a bottoms fraction comprising solvent suitable for reuse in said extraction zone; and,

(e) returning at least a portion of said sidecut fraction of step (d) to step (a) as said added solvent.

2. Method according to claim 1 wherein said solvent comprises sulfolane.

3. Method according to claim 2 wherein said lower stream from the first distillation zone is introduced into the second distillation zone at a locus below the locus for introducing said vapor sidecut stream.

4. In a process for the recovery of aromatic hydrocarbons dissolved in sulfolane solvent wherein non-aromatic hydrocarbons are contaminating the extract phase from the solvent extraction zone, said contaminants being removed by extractive distillation of the extract phase, the improvement which comprises withdrawing from the extractive distillation column a vapor sidecut stream comprising a concentrate of aromatic hydrocarbons and liquid bottoms stream comprising solvent, passing both withdrawn streams into an aromatic recovery distillation column maintained under distillation conditions; withdrawing from the aromatic recovery column a distillate fraction comprising aromatic hydrocarbons, a bottoms fraction comprising lean solvent, and a sidecut fraction comprising solvent containing aromatic hydrocarbons;

and, returning said sidecut solvent fraction to the extractive distillation column.

5. Improvement according to claim 4 wherein said aromatic hydrocarbons are mononuclear aromatic hydrocarbons.

(References on following page) 7 8 References Cited DELBERT E. GANTZ, Primary Examiner UNITED STATES PATENTS C. R. DAVIS, Assistant Examiner 3,222,416 12/1965 Evans et al. US L X 3,338,823 8/1967 Voetter 208231XR 20 321 325 3,361,664 1/1968 Broughton et a1. 5 

